The present invention relates to an ignition timing control system for an engine to control an ignition timing for an initial or a starting period immediately after starting the engine so as to gradually advance the ignition timing from a fixed ignition timing position.
Heretofore, as an example of an ignition timing control system of this kind, there has been disclosed a system, e.g., Japanese patent application laid-open No. 61-96181. This system is based on an angular control system to detect projections or slits provided on a crank rotor rotating synchronously with a crank shaft to measure an ignition timing. Moreover, a system disclosed in Japanese patent laid-open No. 60-47877 is based on a time control system to detect the time elapsed between projections or slits provided on the crank rotor with a predetermined interval to measure the ignition timing.
Meanwhile, as the number of engine revolutions at the time of cranking is unstable, it is required to, set the ignition timing (e.g., at BTDC 10.degree.) at the time of cranking. For this reason, many systems adopt a system to advance an ignition angle after starting of the engine to shift to ordinary ignition timing. Generally, a timing for shifting to such an ordinary ignition timing control is switched to a fixed position in dependency on an increasing rate of the detected engine revolution speed, when a starter switch is switched from an ON to an OFF state.
In an ordinary operating state where the engine revolution speed is stable, ignition timing control by the time is more advantageous than ignition timing control by the angle because of a shortened computing speed and a simplified structure.
To the contrary, for an unstable initial time period immediately after starting, it is difficult to precisely detect changes in the revolution speed.
Namely, as shown in FIG. 1 (a fixed ignition timing period at the time of cranking), where projections 1a and 1b are formed at an outer periphery of a crank rotor 1, e.g., at positions of BTDC (Before Top Dead Center) 10.degree. and BTDC 100.degree., at the time of cranking, an ignition signal is output to driver (not shown) when a crank pulse produced by detecting the projection 1a is output to spark an ignition plug (FIG. 1 shows this state).
On the other hand, as shown in FIG. 2 (an ignition timing control immediately after starting), when the starter switch is turned OFF after complete firing, or when the engine speed rises to a predetermined value, the ignition timing control is switched to ordinary timing control. First, an angular velocity is calculated from a time period between the time when the projection 1a is detected and the time when the projection 1b is detected. Then the angular velocity is converted to an ignition timing (ignition angle) in dependency on the operating state to an ignition time in accordance with the calculated angular velocity. An ignition timing is therefore measured with reference to the time when the projection 1b is detected. When the timing reaches a predetermined ignition timing (BTDC 20.degree. in FIG. 2), an ignition signal is output.
For an initial time period immediately after starting of the engine, engine speed is low and an interval of the time period .alpha. is elongated. When the engine revolution speed for the initial time period varies largely, even if the ignition timing is set at BTDC 20.degree. as shown in FIG. 2, an actual ignition angle may be excessively advanced, e.g., to an extent of BTDC 30.degree..
As a result, combustion after complete firing becomes unstable. Thus, when the ignition timing is suddenly advanced from the fixed ignition timing, the engine revolution speed does not rise smoothly. Consequently, engine stall would occur, and the starting performance becomes unsatisfactory.